The present invention relates to a process for the production of 2-hydroxy-4-methylmercaptobutyric acid, referred to below as methionine hydroxy analog or MHA for short, from 3-methylmercaptopropionaldehyde.
2-Hydroxy-4-methylmercaptobutyric acid is used as a feed additive in a similar way to methionine and, owing to the structural similarity, it is therefore known as methionine hydroxy (MHA) analog.
Up to the present, MHA has conventionally been obtained from 3-methylmercaptopropionaldehyde, which, in turn, is obtainable by addition of methyl mercaptan to acrolein, by reaction with hydrogen cyanide and subsequent hydrolysis of the 4-methylmercapto-2-hydroxybutyronitrile formed. The need to use hydrogen cyanide is a disadvantage of this process. Owing to the high toxicity of hydrogen cyanide, costs relating to safety must be high for the reaction. Another very great disadvantage is the ammonium salt formed by the introduction of nitrogen and its subsequent hydrolytic cleavage, which is formed stoichiometrically and causes correspondingly high pollution of waste water. There is therefore a need for an HCN-free process for the production of MHA.
Accordingly, an object of the present invention is to provide a novel process for the production of MHA, in which, on the one hand, methylmercaptopropionaldehyde is used as a starting component and, on the other hand, instead of HCN another C1 building block is to be reacted with methylmercaptopropionaldehyde (MMP).
It is known (EP-A 0 189 120 and G. Silvestri et al., Tetrahedron Letters 1986, 27, 3429-3430) to react carbon dioxide as a C1 building block electrochemically with ketones and aldehydes, with xcex1-hydroxycarboxylic acids being formed. While the electrochemical carboxylation of aromatic ketones generally leads to average to good yields, only moderate yields are achieved in the electrochemical carboxylation of aromatic aldehydes and in the carboxylation of aliphatic aldehydes, indeed, only low yields are achieved. In the process of the documents evaluated above, the electrocarboxylation takes place in an undivided electrolytic cell, which contains a sacrificial anode, in an aprotic solvent, which additionally contains a supporting electrolyte. The low yields and low selectivities of the electrochemical carboxylation of aldehydes, and especially aliphatic aldehydes, that have become known up to the present have, until now, prevented a person skilled in the art from seriously considering this method for an industrial process, such as the electrocarboxylation of 3-methylmercaptopropionaldehyde with CO2.
Against all expectations, it has now been found that MMP can be carboxylated electrochemically in a high yield. The present invention accordingly provides a process for the production of 2-hydroxy-4-methylmercaptobutyric acid (MHA), which is characterized in that 3-methylmercapto-propionaldehyde (MMP) is electrochemically carboxylated with carbon dioxide in an undivided electrolytic cell containing a sacrificial anode in an aprotic solvent in the presence of a supporting electrolyte at an effective cell voltage and MHA is obtained from the MHA salt formed, which is dissolved and/or suspended in the electrolyte and the cation of which comes from the anode.
The process according to the invention is carried out in a simple electrolytic cell, which has only a single electrolyte chamber, as understood by the term xe2x80x9cundividedxe2x80x9d.